Chain drive tensioner spring force control mechanism

Abstract

A tensioner which adjusts the mean tensioner force to keep the chain tension as low as possible without sacrificing chain control, significantly improving drive efficiency as the chain wears and is subject to low dynamic loads.

What is claimed is: 
     
       1. A tensioner for a passive tensioner system for tensioning a chain span or a belt, comprising:
 a housing having a cylindrical bore; 
 an external piston coaxially arranged within the cylindrical bore of the housing, comprising a body having a first diameter portion, a second diameter portion having a smaller diameter than the first diameter portion, a transition surface between the first diameter portion and the second diameter portion, an open end and a closed end, a bottom surface at the open end, and a hollow interior having an inner diameter; 
 an internal piston received within the hollow interior of the external piston, comprising a body having a first end and a second end; 
 an external piston spring received within the hollow interior of the external piston, for exerting an outward bias force on the external piston outwards from the housing, the external piston spring having a first end contacting an inner surface of the closed end of the external piston and a second end contacting the first end of the internal piston; 
 an internal piston spring received within the cylindrical bore of the housing for biasing the internal piston outwards from the housing, the internal piston spring having a first end in contact with the second end of the internal piston and a second end in contact with an end of the cylindrical bore of the housing; 
 a first hydraulic chamber formed by the cylindrical bore of the housing, the internal piston, and the bottom surface of the external piston, having a first fluid input; such that an inward force acting to push the external piston into the housing creates a fluid pressure in the first hydraulic chamber, causing the internal piston to move relative to the external piston, changing the outward bias force of the external piston spring on the external piston; and 
 a second hydraulic chamber formed by the cylindrical bore of the housing and the transition surface of the external piston, having a second fluid input with an inlet check valve, such that fluid pressure in the second hydraulic chamber exerts an outward force on the external piston, the inlet check valve opposing fluid flow from the second hydraulic chamber when an inward force is exerted on the external piston. 
 
     
     
       2. The tensioner of  claim 1 , wherein the cylindrical bore comprises a first portion and a second portion, the first portion having a diameter smaller than a diameter of the second portion. 
     
     
       3. The tensioner of  claim 2 , wherein the first diameter portion of the external piston is received by the first portion of the cylindrical bore and the second diameter portion of the external piston is received by the second portion of the cylindrical bore. 
     
     
       4. The tensioner of  claim 1 , further comprising a pressure relief valve in fluid communication with the second hydraulic chamber. 
     
     
       5. The tensioner of  claim 1 , further comprising a third chamber at atmospheric pressure, the third chamber being formed between the first end of the internal piston and the hollow interior of the external piston. 
     
     
       6. The tensioner of  claim 1 , wherein the first fluid input further comprises a check valve. 
     
     
       7. The tensioner of  claim 1 , wherein the internal piston is hollow having an interior with an inner diameter, the first end of the body is a closed end, the second end of the body is an open end; and the open end having a bottom surface end. 
     
     
       8. The tensioner of  claim 7 , wherein the cylindrical bore comprises a first portion and a second portion, the first portion having a diameter smaller than a diameter of the second portion.